This invention relates to the noninvasive measurement of a patient's blood characteristics and, in particular, to the use of a pulse oximeter to measure the patient's heart rate and blood oxygen saturation.
The use of pulse oximeters to noninvasively measure a patient's heart rate and arterial blood oxygen saturation is well known. In general terms, noninvasive measurement of arterial blood oxygen saturation typically requires the transcutaneous illumination of a portion of the patient's blood-perfused tissue by light and two or more wavelengths. Changes in the amount of arterial blood in the tissue during a blood pressure pulse change the amount and character of the light detected by the sensor's photodetector. Pulse oximeters identify arterial blood (as opposed to venous blood) from the pulsing component of the optical signal detected by the pulse oximetry sensor. The amounts of light transmitted through the tissue at each wavelength may be compared to calculate to what degree the arterial blood flowing through the tissue is saturated with oxygen. A more detailed discussion of the principles of pulse oximetry may be found in U.S. Pat. No. 4,653,498.
The quality of the pulse oximetry measurement depends in part on the concentration of arterial blood (relative to other tissue structures) in the portion of tissue illuminated by the sensor and in part on the magnitude of the pulsatile changes in the amount of blood in the tissue. For example, while fingers are a preferred sensor site because of their relatively large number and concentration of blood vessels, well-perfused sites such as fingers are not always available. In addition, blood flow to the sensor site may be restricted due to the effects of ambient temperature, systemically acting vasoconstricting drugs in the patient's blood stream, or low patient blood pressure.
The prior art has recognized a need to increase the amount of arterial blood. A device called the "Cyclops" is discussed in W. G. Zijlstra and G. A. Mook, Medical Reflection Photometry, P. 50-77 (Royal VanGorcum Ltd., Assen, 1962). The Cyclops detects blood by measuring the amount of light reflected from the skin and is preferably placed in the middle of the forehead, thus giving it the name "Cyclops". The skin is first given a treatment of histamine phosphate. This is a counterirritant which has the effect of producing vascular dilation. With the blood vessels dilated, more blood will flow through, providing a larger signal to the sensor.
A voltage is applied to the skin to drive the histamine phosphate into the tissue by a process called histamine iontophoresis. Subsequently, a compression plate is applied to the skin to make it bloodless to enable a base line measurement of the light reflection value. Thereafter, blood is allowed to flow to dilated vessels and measurements are taken of the blood oxygen level.
In more recent years, pulse oximeter devices have been developed which use multiple light wavelengths and sophisticated waveform analysis which allows a pulse waveform to be analyzed at its peaks and valleys, eliminating the need for making a bloodless base line measurement. These devices have attempted to provide a simple, compact sensor which can provide all the necessary preparation and analysis with a single application of the sensor, without the need for the multiple steps of the prior art Cyclops. For example, U.S. Pat. No. 4,926,867 discloses a pulse oximetry sensor that incorporates a heater and a temperature control. That patent states that the application of heat to the patient's skin at the sensor site increases the flow of arterial blood through the capillaries beneath the sensor, thereby increasing the pulse to pulse changes in the optical signal derived by the sensor.
U.S. Patent No. 4,825,879 discloses another prior art approach to the blood perfusion problem. In that patent, the inventors recognize that the addition of a heater to the sensor increases its complexity and makes it more costly. The inventors therefore added a thermally reflective metal layer to the pulse oximeter sensor in order to retain body heat at the sensor site.